Tim Marshall

Diplomatic Editor

The focus of the final TV debate plays to both US President Barack Obama's strengths and weaknesses in foreign affairs.

His grasp of detail can be expected to be better than Governor Mitt Romney, and he can point to ending US military involvement in Iraq, and the killing of Osama Bin Laden as successes.

He is less likely to bring up the subject of drone attacks in Pakistan.

He has quadrupled the number of missile strikes since taking over from President George Bush.

Mr Romney knows better than to attack Mr Obama’s strengths and will go for what he perceives to be weak spots.

Foremost among those will be the murder of Ambassador Chris Stevens last month in Benghazi.

Image:Ambassador Stevens died on September 11 in an attack on the US consulate

The Republicans are blaming the Democrats for putting Mr Stevens and his staff in danger by not supplying adequate safety measures.

They also accuse the Obama administration of lying for saying at first that the incident was a demonstration which had got out of hand, and only later admitting it was a planned terrorist attack.

Mr Romney came a little unstuck in the second debate when he was informed Mr Obama had called it a terrorist attack the day after the incident. But the overall charge remains and, if he chooses his words more carefully, Mr Romney could go on the offensive over the issue.

The overall situation in Libya does not benefit Mr Obama - the country remains in chaos and heavy fighting continues in the southern town of Bani Walid.

This plays into the Romney narrative that Mr Obama has shown 'passivity' in the face of Islamist militancy.

He will argue that Mr Obama 'reaching out' to the Muslim world has been seen as weakness by America's enemies.

Mr Obama can counter that it is in America's interest to deal with reality, which is that free elections in the Arab countries will result in Islamist parties taking power, and therefore the US has to engage with them.

It is likely Mr Obama's quote that Egypt, under Muslim Brotherhood rule, is now 'neither an ally nor an enemy' will be thrown back at him.

The Romney camp says it will use America's $1.5bn of annual aid to ensure that Egypt is either an ally, or does not benefit from American tax payers' dollars.

Image:Syria's President Assad has resisted pressure to stand down

That is an argument Mr Romney applies to other Arab countries to which Mr Obama's rejoinder is that a Romney approach would risk ushering in the collapse of countries to America's detriment.

Syria is a problem for both men. For more than a year, Mr Obama has called for Syrian President Bashar al Assad to stand down, but has done little to ensure it happens.

The State Department continues to struggle to tell him which of the many anti-Assad factions he should be supporting, and he stands accused of encouraging Syrians to rise up against Mr Assad, and then doing little to help.

Mr Romney will go on the offensive in Syria, but needs to tread a cautious line.

The American public, weary of wars in Muslim countries, may not take kindly to hints that the US would become embroiled in another.

Which brings us to Iran: Both men will say they will not allow an Iranian nuclear bomb on their watch. The question is how they will prevent it.

If Mr Obama repeats his 'open hand' offer, he risks looking weak. If Mr Romney verbally fires up the cruise missiles, he will frighten many voters.

And the Israel/Palestine issue? That is another Obama weak point. He has had zero progress in four years, and there is no prospect of any on the horizon despite having said four years ago that it would be a foreign policy priority.

Of course, there is a lot more: the 'pivot' to a China-centric policy, the 'war on drugs' in Latin America, the endgame in Afghanistan.

There's also the economy. This may be foreign policy, but neither man is likely to be able to resist a few remarks on what is the central issue in the campaign.

Unless someone drops a verbal bombshell, the foreign policy debate is likely to be won and lost on nuance.

The undecided voter is often swayed by the thrust of an argument, not necessarily the detail.

Mr Obama speaks softly, with an occasional reminder that there is a big stick in the armoury.

Mr Romney brandishes the stick, with the occasional reminder of carrots.